Water/oil resistant paper is sometimes used as a wrapping material for food products, such as wrapping paper for food products or a packaging container for food products. The water/oil resistant paper is produced by applying a water/oil resistant agent on a substrate such as paper or by treatment such as addition of the water/oil resistant agent to a pulp slurry.
For example, some water/oil resistant agents contain a polymer. As the polymer, a vinyl polymer having hydrophobic groups and hydrophilic groups as pendant groups is widely used, since such a vinyl polymer has water/oil resistant property and is excellent in solubility and dispersibility in water.
An organic solvent is usually used as a polymerization solvent for producing the vinyl polymer. In order to use the vinyl polymer as raw material for the water/oil resistant agent in the application for food products, it is necessary to remove an organic solvent from an organic solvent solution after the polymerization so as to be a water dispersion.
As the method for removing an organic solvent from a vinyl polymer dispersion containing the organic solvent and water, a method has been known that the dispersion is heated from the outside of a closed container by a jacket, whereby the organic solvent is evaporated and removed. For example, in Patent Document 1 at Examples 1 to 3, the outside of a reaction container is heated by jacket at from 50 to 65° C. under the condition of reduced pressure, whereby the inside of a closed container is heated to remove acetone.